【Daily Question 20090316】Types and functions of safety barriers

Types of safety barriers: 1. Zener-type safety barriers 2. Isolated safety barriers. Function of safety barriers: Intrinsic safety barriers are used in the design of intrinsic safety explosion-proof systems. They are devices installed in safe areas and contain both intrinsic safety circuits and non-intrinsic safety circuits. Current-limiting and voltage-limiting circuits are used in these circuits to restrict the amount of energy supplied to the intrinsic safety circuit, thereby preventing dangerous energy from the non-intrinsic safety circuits from reaching the intrinsic safety circuit. In intrinsic safety explosion-proof systems, they are referred to as associated equipment and constitute an important part of such systems. The safety barrier is installed in a safe area; it receives signals from the hazardous area and outputs safe signals to either the safe area or the hazardous area. 1. Zener-type safety barriers use fast fuses, current-limiting resistors, or diodes in the circuit to limit the electrical energy input, thereby ensuring the amount of energy delivered to the hazardous area. Its principle is simple, its circuit implementation is easy, and it is inexpensive; however, its reliability in practical applications is affected due to inherent design flaws. The reason is that the installation location must have a highly reliable grounding system, and the grounding resistance of this Zener safety barrier must be less than 1 ohm; otherwise, its explosion-proof protection function will be lost ； It has a significant impact on the power supply, and fluctuations in the power supply can also cause damage to the Zener safety barrier. 2. Isolated safety barrier: The isolated safety barrier employs a circuit structure that provides electrical isolation between the input, output, and power supply, while also meeting the requirements for energy limitation in intrinsically safe systems. Compared to Zener-type safety barriers, although it is more expensive, its outstanding performance advantages bring greater benefits to users’ applications. No grounding required ； The requirements for the instruments are not high; they offer good stability and strong interference resistance, thereby enhancing the security of the system ； It has strong signal processing capabilities ； It can output two mutually isolated signals. This post was last edited by Meihua Piaoxiang on 2009-3-16 10:59.]

A Zener safety barrier operates based on the reverse breakdown property of Zener diodes. The functions of voltage limiting and current limiting. Isolated safety barriers incorporate functions such as voltage limiting, current limiting, and isolation. Major manufacturers: P+F, MTL, TURCK

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A Zener safety barrier operates based on the reverse breakdown property of Zener diodes. A Zener safety barrier consists of a voltage limiting circuit, a current limiting circuit, and fuses, etc. The function of the voltage limiting circuit is to keep the voltage on the safety circuit side at the rated operating voltage. When the supply voltage V1 equals the rated operating voltage of the Zener safety barrier, neither Zener diode DW1 nor DW2 conducts; at this point, the voltage V2 on the safe spark circuit side equals the power supply voltage required by the field instruments. When the supply voltage V1 exceeds the rated operating voltage, the Zener diode DW1 breaks down and conducts first; as a result, the voltage V2 on the safety spark circuit side remains unchanged. When V1 rises to equal the maximum safe holding rating voltage, the current flowing through DW1 and DW2 causes the Zener diodes to heat up and get damaged; at this point, the fuse RD blows, cutting off the power supply and thereby providing protection for the safe spark circuit side. The fuse blow time Tf and the Zener diode blow time TZ must satisfy the requirement of 10Tf < TZ. In the Zener safety barrier, the use of thick-film fast-blow fuses ensures that the maximum safe operating voltage for the voltage V1 on the non-safe spark circuit side is 340V DC. The function of the current limiting circuit is to limit the current on the safe spark circuit side to below 35mA DC. For the Zener safety barrier used in conjunction with the transmitter, as long as the transmitter’s output current is within the range of 4–20 mA DC, the safety barrier has no impact on the operation of the transmitter. However, in the event of a fault in the transmitter, such as a short circuit, the current-limiting circuit limits the current on the safe spark circuit side to below 35 mA DC. Overview of Applications for Isolated Safety Gates: In industrial environments, two-wire transmission systems are generally used for power distribution units. These units are required to supply a 24V power supply to primary instruments such as pressure transmitters, while also collecting, amplifying, processing the incoming current signals, and filtering out interference before outputting isolated current and voltage signals for use by subsequent secondary instruments or other devices. However, some special industrial installations require not only two-wire transmission to provide both power supply and signal isolation, but also explosion-proof properties of the safety spark type to reliably prevent contact between high voltage from the power supply and the signals. These installations need special-purpose distributors known as safety barriers, which utilize dual control of current and voltage to limit the energy entering hazardous areas to levels below safe thresholds. Isolated safety barriers basically come in two types: detection-side safety barriers and operation-side safety barriers. The detection-side safety barrier is used in conjunction with two-wire transmitters ； The operational safety barrier is used in conjunction with electrical converters or electrical valves. There are also isolated safety barriers of types such as signal input. Due to the use of measures such as voltage and current limiting as well as isolation in isolated safety barriers, it is not only possible to prevent dangerous energy from entering the hazardous area through intrinsically safe terminals, thereby enhancing the system’s intrinsically safe explosion-proof properties, but it also improves the system’s resistance to interference, **increasing the reliability of the system’s operation. Working principle: The 24VDC power supply is converted through DC-AC-DC conversion to generate various voltages required by the module circuit. The principle of the isolated safety barrier at the detection end is as follows: The modular circuit converts the current or voltage signals input through the intrinsically safe energy limiting circuit into values between 0.2 and 1 VDC. These signals are then fed into the module where they are collected, amplified, processed to reduce interference, and subsequently transformed by a transformer into isolated current and voltage signals that can be used by subsequent secondary instruments or other devices. The module must also output a isolated 18.5∽28.5VDC voltage, which is used via an intrinsically safe energy limiting circuit as the operating voltage for two-wire transmitters. The intrinsically safe energy limiting circuit can prevent dangerous signals with high current or high voltage from entering hazardous areas. The principle of the isolated safety barrier at the control side is to isolate the 4-20mA DC signal output by the regulator or operator, and then output another 4-20mA DC signal, which is supplied through an intrinsically safe energy limitation circuit to the electrical converter or the on-site electrical valve positioner for use. The intrinsically safe energy limiting circuit can prevent dangerous signals with high current or high voltage from entering hazardous areas.